The present invention relates generally to a hydraulic control system for distributing fluid to a plurality of fluid actuated devices and more particularly to such a hydraulic control system for use with a variable displacement pump.
It is frequently necessary for a single hydraulic pump to provide sufficient hydraulic fluid to operate a plurality of fluid actuated devices. For many years fixed displacement pumps have been utilized in combination with open center control valves to distribute the fluid to the desired fluid actuated device. In those instances where the control valve was placed in a neutral position, providing no distribution of fluid to the fluid actuated device, these systems were subjected to excessive flow pressure, power losses and heat dissipation problems. Additionally, in those instances where the fluid actuated devices utilize only a portion of a fixed pump displacement, the remainder of the pump power is wasted.
To overcome these difficulties variable displacement pumps have been used in combination with closed center control valves to achieve better system efficiency. In some applications the variable displacement pumps have been provided with pressure compensated controls. In these applications the pump idles at maximum system pressure. As a result, the control valve must meter down to the actual load pressure, which may provide a very large pressure drop across the valve. Such large pressure drops also result in a very inefficient use of energy.
In still other applications variable displacement pumps in combination with a closed center control valve have been utilized in a load sensitive system. Such a system requires the use of a flow demand control valve for sensing the differential pressure between the pump inlet and some controlled point in the fluid supply to the load. Such load sensitive systems suffer from the disadvantage of being quite complex and therefore expensive to manufacture. Additionally, such load sensitive systems may be fooled in those circumstances where a load is being lowered with the aid of the force of gravity. Under these circumstances the load signal may reverse, resulting in diminished pump output rather than the desired increase in pump output.
Many of the aforementioned problems are discussed and solved by the type of control system disclosed in U.S. Pat. No. 3,788,077 to Johnson, et al. This patent shows a hydraulic control system for a variable displacement pump including a standard open center type of control valve, and a standard fluid actuated displacement control mechanism for the pump. A control pressure signal indicative of pump output pressure is developed in the control valve and this control pressure signal is utilized to control the position of a sensor valve. The sensor valve in turn controls communication between the pump and the displacement control mechanism and communication between the displacement control mechanism and a reservoir. Systems of this type have generally lacked the responsiveness required for the control of many fluid actuated devices.